For example, JP-A-2011-112569 discloses an airflow measuring device. As shown in FIG. 5, a conventional airflow measuring device 100 includes a housing 105 and a flow rate sensor 106. The housing 105 has a bypass passage 102 and a sub-bypass passage 103. The bypass passage 102 receives a part of air flowing through a duct 101. The sub-bypass passage 103 receives a part of the air branched from the bypass passage 102. The flow rate sensor 106 is provided in the sub-bypass passage 103 to detect the flow rate of air flowing through the sub-bypass passage 103. The configuration of the branch from the bypass passage 102 into the sub-bypass passage 103 separates dust included in the air flowing through the duct 101 thereby to flow the dust into the bypass passage 102.
In the configuration of FIG. 5, the bypass passage 102 extends straight along the airflow passing as a mainstream in the duct. Dust contained in air passing through the bypass passage 102 moves straight with inertial force thereby to be separated at the branch from the bypass passage 102 into the sub-bypass passage 103.
It is noted that, in the conventional airflow measuring device 100, the passage wall surface of the bypass passage 102 around its outlet is partially directed toward a branch 108 between the sub-bypass passage 103 and the bypass passage 102. In such a configuration, as shown by the two-dot chain arrow in FIG. 5, dust may rarely repeat collision against the passage wall surface to result in reaching the branch 108. In this case, the dust may consequently intrude from the branch 108 into the sub-bypass passage 103.